Quantifying vertical wind shear effects in shallow cumulus clouds over Amazonia

Abstract

Abstract. This study analyses and quantifies the effects of vertical wind shear (VWS) on the properties of shallow cumulus cloud fields over Central Amazonia. We perform idealized simulations with high resolution (50 m horizontally, 20 m vertically) using the Dutch Atmospheric Large Eddy Simulation (DALES) model, changing the initial conditions and large scale forcing of VWS. The resulting cloud field is analysed with by applying a cloud tracking algorithm to generate Lagrangian datasets of the lifecycle of individual clouds as well as their time-varying core and margins dimensions. The reference run has no wind speed or directional shear and represents a typical day in the local dry season. Numerical experiments with moderate and high wind speed shear are simulated by adding linear increases in the wind speed of 1.2 m s−1 km−1 and 2.4 m s−1 km−1, respectively. Three additional runs are made by adding 90° wind rotation between the surface and the top of the domain (5 km) on top of the three wind speed shear conditions. We find that clouds developing in a sheared environment have horizontal equivalent diameter increased by up to 100 m on average, but the cloud depth is reduced. Our quantification shows that VWS tends to increase the size of the cloud cores, but reduce its relative area, volume, and mass fractions compared to the overall cloud dimensions. The addition of 2.4 m s−1 km−1 of VWS decreases the relative core area by about 0.03 (about 10 % of the overall average) and its volume and mass ratios by about 0.05 (10 %–25 % in relative terms). Relevant for the cloud transport properties is that the updraught speed (w) and the liquid water content (LWC) are lower within the cores, and consequently the upward mass flux. All quantifications of cloud properties point to the inhibition of convective strength by VWS, therefore hampering the shallow-to-deep transition. However, open questions still remain given that a the individually deepest clouds were simulated under high environmental shear, even though they occur in small numbers. This could indicate other indirect effects of VWS that have opposite effects on cloud development if found to be significant in the future.